Sometimes your favorite stainless steel travel mug will stop keeping your beverages warm or cold for no apparent reason. This may occur as a slow process that spreads out over several weeks or it may occur suddenly with the mug working fine one day and not the next. So what happens and why does this occur? The answer is fairly simple when you think about the way that high quality mugs are manufactured and insulated.

There are several different ways that manufacturers use to insulate a travel mug. The most important of these is the vacuum insulation system. What this means is that the manufacturer makes the vessel portion of the mug in two pieces. These amount to an inner shell and an outer shell. The shells are very similar in size, but the inner shell is just a little bit smaller than it has to be in order to fit inside the outer shell. If you set one water glass down inside another you may notice a small empty space between the bottom of the glasses and perhaps between their sides. This is the same principle that mug manufacturers are using when they build a vacuum insulation system.

Once the two shells have been assembled they are sealed together around the top. This leaves a small open space between the two shells, and to insulate the mug the manufacturer will draw all of the air out of this empty space, leaving a vacuum that is held in place between the two shells by the seal where the shells are joined at the top. It is the vacuum in this empty space that actually insulates most stainless steel travel mugs.

For the most part this is an extremely durable way to put together a travel mug that is both lightweight and highly insulated, but unfortunately the seals that hold the vacuum in place will occasionally leak. When this happens the travel mug will lose its insulation ability, and any contents held within the mug will begin to rapidly warm or cool as the case may be.

Leakage of this vacuum seal may sometimes be caused by a sudden drop, especially one around the unprotected rim where the seal is located. More often it just seems to occur usually as the mug ages, and may be a result of many hot cold cycles as hot beverages are placed inside and consumed.

If you suspect your travel mug may have a vacuum seal leak because it suddenly has lost its insulating ability, there isn’t really much you can do to repair or fix the issue. The mug can still be used it just won’t have the ability to keep your beverages warm or cold.

You can check for leaks by submerging the mug in a sink full of water for several minutes to an hour. Then empty the mug completely and hold it up side down to see if you can find a small drip from somewhere around the rim. You may need to turn the mug slowly so that each part of the rim as an opportunity to point downward. If you use this method please remember that plastic covers and handle attachments can sometimes hold water or other moisture and the drip you see may be coming from nothing more than the handle or thread fittings.

Another thing you might try after you submerge the stainless steel travel mug for a while, is to drain it and then shake it near your ear to see if you can hear water sloshing. The idea with both of these methods is that a broken seal that allowed air into the space normally containing a vacuum will also allow water to leak in if you submerge the mug long enough.